1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and an image processing program, in which an order of printing is controlled based on a difference in transfer efficiency between a first face and a second face of a printing medium after duplex printing, and a storage medium storing the image processing program.
2. Description of the Related Art
Some recent image forming apparatuses such as copiers and printers are known to perform duplex printing in which images are formed on both of a first face (hereinafter, is also referred to as a “front face”) of a printing medium and a second face (hereinafter, is also referred to as a “back face”), which is an opposite face of the first face. Duplex printing is performed in such a manner that after an image is transferred and fixed to a first face of a printing medium, a printing medium reversing section or the like reverses the faces of the printing medium to supply the printing medium again to a transfer section on a transport belt, and then an image is transferred and fixed to a second face. After the duplex printing, for example, the printed materials can be stapled on the left side to be formed into a booklet.
When the duplex printing as described above is performed, a difference in fog or image density occurs between the first face and the second face in the printing medium after the duplex printing. Thus, the following problem is caused. FIG. 8 is a view showing the relationship between the order of printing in a conventional image forming apparatus and the page number of a formed booklet. FIG. 9 is a schematic view showing a state in which the booklet formed after the duplex printing performed by the conventional image forming apparatus is open. For example, as shown in FIG. 8, eight document images having an equal background density arranged in the order A, B, C, D, E, F, G, and then H are read by an image reading section of the image forming apparatus in the order A, B, C, D, E, F, G, and then H, and a series of pieces of image data is generated. Then the image data is printed by duplex printing on first faces and second faces of four sheets of printing medium in the order of printing that is the same as the order of reading of the document images, and then the obtained printing materials are stapled on the left side to be formed into a booklet. In the thus formed booklet, as shown in FIG. 9, the second page and the third page, the fourth page and the fifth page, and the sixth page and the seventh page, that is, two facing right and left pages are obtained as the combination of a first face R and a second face W of the printing medium. Thus, an outstanding difference in fog or image density may occur between the left and right pages. Such a difference in fog or image density appears particularly significantly with document images in which the color tone of the background is halftone.
In order to solve such a problem, for example, in an image forming apparatus in Japanese Unexamined Patent Publication JP-A 2004-357074, different density samples printed on a front face and a back face of a document are read by line sensors provided respectively on the front face and the back face, and parameters in image processing are corrected such that the densities on both faces are equal to each other, based on the difference in density between printed materials of the data read with the line sensors.
As described below, there are two causes of the difference in fog or image density between a first face and a second face of a printing medium after duplex printing. A first cause is that an image on a front face and an image on a back face of a document are read by different line sensors, and thus a difference in reading density between individual line sensors appears as the difference in fog or image density.
Furthermore, a second cause is that a resistance value of a printing medium in image formation on the second face is higher than that on the first face. This is caused by the phenomenon that when a toner image is fixed to the first face by thermal fixing, water in the printing medium evaporates due to heat applied in the fixing, and the amount of water in the printing medium decreases. With such different resistance values of the printing medium, the transfer efficiency in image formation on the first face is different from that on the second face, and thus the difference in fog or image density appears.
According to the image forming apparatus in JP-A 2004-357074, it is possible to reduce the difference in fog or image density derived from the first cause, that is, the configuration in which an image on the front face and an image on the back face of a document are read by different line sensors. However, this image forming apparatus is for correcting parameters in, for example, development conditions or surface potential conditions, and thus it is not possible to reduce the difference in fog or image density derived from the second cause, that is, the difference in transfer efficiency between image formation on the first face and image formation on the second face.